2.39.1 Shaft Driven Compressor (SDC).
The SDC is
mounted on the main transmission accessory case and
uses outside air. The air passes through a particle separa-
tor and then through a butterfly-type throttle valve. This
valve is closed by a time-delay solenoid when the APU
START switch is engaged. The valve remains closed for
60 seconds after the APU START switch is released. The
purpose of the time-delay solenoid is to reduce starting
loads on the APU during external power starts. It is not ac-
tivated during a battery start. The shaft driven compressor
will compress air at 3:1 ratio and route it through a surge
valve. This valve, mounted on top of the SDC, will open or
close as necessary to keep SDC air pressure constant
with variable system demand. Pressurized air then
passes through a spring-operated check valve and into
the PAS manifold. This check valve prevents pressurized
air, from sources other than the SDC, from reverse routing
back to the SDC. The SDC is lubricated by the main trans-
mission accessory oil pump and has its own scavenge. If
the SDC oil temperature exceeds 182 C (360 F), or if the
SDC fails, the pilot SHAFT DRIVEN COMP caution light
2.39.2 No. 1 Engine Bleed Air.
The No. 1 engine pro-
vides an alternative source of pressurized air in case of
SDC failure. The engine compressor bleed air selector
valve opens automatically if SDC output falls below 10 psi.
Simultaneously, a spring-operated check valve opens.
When closed, this valve prevents pressurized air from go-
ing to the engine compressor when the SDC is operating.
Each engine provides bleed air to its engine inlet to pre-
vent ice formation and to keep the engine cooling louvers
closed while the engine is operating. The bleed air used in
these last two instances is independent of the pressurized
2.39.3 External Air Source Receptacles.
tacle for external air source connection (fig 2-46) is on the
lower portion of No. 1 engine nacelle. When the air source
is connected and operating, a normally closed check
valve to the pressurized air system opens to allow air to
pressurize the manifold.